1. Field of the Invention
The present invention relates to a high production chill roll. More particularly, the present invention relates to a chill roll for cooling a web of material in the extrusion coating of paper and plastic film extrusion. The chill roll permits production rates which are higher than previously attainable by reducing the retention of heat in the outer shell of the chill roll. Furthermore, the chill roll of the present invention distributes coolant material throughout the inside of the chill roll to achieve uniform cooling across the surface of the chill roll.
2. Description of the Prior Art
Conventional chill rolls are used, in the formation of rolls of plastic and paper-based products, to cool the web of material passing around the surface of the chill roll. Commonly, water is passed through hollow cylinders, as the web of material passes around it. The production capacity of a chill roll is determined by a number of factors, including but not limited to: the diameter of the chill roll, the speed of rotation, the thickness of the outer surface of the roll, the effectiveness of the coolant material, and the uniformity of the cooling of the chill roll surface.
In most chill rolls water enters one end of the roll and exits the opposite end. As the water moves along the length of the chill roll it draws heat from the web of material traveling around the chill roll. Consequently, the side of the web closest to the water inlet is cooled to a much higher degree than the side of the web nearest the water exit. Such a temperature gradient across the chill roll lowers the production capacity of the system or produces inferior quality products.
Extrusion coating for applying plastic to paper requires chill rolls, or some means, to produce larger cooling capacity per unit area. The higher cooling capacity is due to the fact that plastic coatings, such as polyethylene, must be cooled to a temperature close to room temperature before it can be stripped from the chill roll. Furthermore, the diameter of the chill roll can not be so large that it creates a displacement of the extrusion die too far from a combining nip, which would cause a loss of coating adhesion, excessive neck-in of the coating, and other difficulties.
While the thickness of the outer shell of the chill roll can be made thinner to assist in the cooling of the web of material, such a reduction brings with it a reduction in the strength of the chill roll. At high speeds and high nip loads such a roll breaks down upon itself.